The present invention generally relates to electrochemical cells, i.e., batteries, and, more particularly, to a primary alkaline zinc/manganese dioxide electrochemical cell having a pressure relief mechanism formed in the cell can for venting when exposed to excessive pressure.
Conventional alkaline electrochemical cells generally include a cylindrical steel can having a closed bottom end, an open top end, and a cylindrical side wall extending between the top and bottom ends. Contained within the can is a positive electrode, referred to as the cathode, which typically comprises manganese dioxide as the active material. Also contained within the can is a negative electrode, referred to as the anode, which typically comprises zinc as the active material. In bobbin-type cell constructions, the cathode is typically formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. In jellyroll-type cell constructions, the anode and cathode are spirally wound. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a collector and seal assembly, which typically includes an annular polymeric seal, an inner metal cover, a current collector, and an outer cover, provides closure to the open top end of the steel can to seal the active electrochemical materials in the sealed volume of the can.
Cylindrical alkaline electrochemical cells are commonly sealed closed by placing the annular polymeric (e.g., nylon) seal in the open end of the steel can and crimping the upper end of the can radially inwardly and over the outer periphery of the seal to compress the seal against the can. However, electrochemical cells commonly employ electrochemically active materials, such as zinc, which generate hydrogen gas and other gases during storage and sometimes during or following service use. With the cell can sealed closed, excessive build-up of high pressure gases within the sealed can may cause damage to the cell and/or the device in which the cell is employed. Thus, it is desirable to provide a controlled vent mechanism that vents highly pressurized gases from within the can to prevent the pressurized gases from reaching extreme levels that may cause the can to uncrimp and release electrolyte solution and particulate matter.
One widely acceptable approach to venting excessive pressurized gases has included the use of a vent formed in the annular polymeric seal of the collector and seal assembly, which is intended to rupture upon experiencing an excessive pressure build-up within the sealed volume of the cell. For example, U.S. Pat. No. 5,667,912 discloses a current collector assembly having a seal with a thinned portion which is intended to shear when the internal pressure exceeds a predetermined pressure limit. Conventional ventable seals of this type typically employ a circular thinned region formed in the annular polymeric seal. However, the amount of space generally occupied by the annular seal and the corresponding current collector, inner metal cover, and outer cover, is significant. The greater the space occupied by the seal and collector assembly, the less space that there is available within the cell for the electrochemically active materials. Consequently, a reduction in the amount of electrochemically active materials provided within the cell generally results in a shorter service life for the cell. It is therefore desirable to maximize the internal volume within the electrochemical cell that is available for the electrochemically active components.
In order to minimize the space occupied by the collector and seal assembly, it has been proposed to form the pressure release mechanism in the closed bottom end of the cell can such that it is covered by the positive contact terminal, as disclosed in U.S. patent application Ser. No. 09/293,225, filed on Apr. 16, 1999, entitled xe2x80x9cBATTERY CONSTRUCTION HAVING PRESSURE RELEASE MECHANISMxe2x80x9d and U.S. patent application Ser. No. 09/443,573, filed on Nov. 19, 1999, entitled xe2x80x9cBATTERY HAVING PRESSURE RELIEF MECHANISM FORMED IN CONTAINER,xe2x80x9d the disclosures of which are hereby incorporated by reference. According to this proposed approach, the positive contact terminal includes an outwardly protruding nubbin having an upstanding wall extending from a peripheral flange that is welded to the closed bottom end of the cell can. Typically, a jacket (e.g., label) is formed around the outer cylindrical wall of the can and extends over the peripheral edge of the peripheral flange of the positive contact terminal. While this approach generally reduces the amount of volume that is otherwise necessary for the collector and seal assembly, and therefore enhances the amount of usable sealed volume within the cell, a number of shortcomings may be present. It is possible that when the pressure relief mechanism vents the positive contact terminal may become separated from the closed bottom end of the can. In addition, undesirably large variability may exist in the range of vent pressures, which may result in unexpectedly high vent pressures. This may be due, at least partially, to the cover being welded to the can at a location such that the cover restricts bulging of the bottom end of the can. In addition, the jacket, which is typically shrink-wrapped onto the outer surface of the can, may also restrict the bulging of the bottom end of the can. As a consequence, variances in the vent pressure and prevention of cover separation may occur.
Accordingly, it is therefore desirable to provide for an electrochemical cell having a vent formed in the closed end of the can with a cover such that the cell is able to consistently vent pressurized gases, and further which is not susceptible to separation between the cover and the can.
The present invention improves the protective safeguards of an electrochemical cell with an enhanced pressure relief mechanism formed in the closed end of a cell container that allows for the use of a low profile seal assembly. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described herein, the present invention provides for an electrochemical cell comprising a container having a first end, a second end, a side wall extending between the first and second ends, and an end wall extending across the first end. The cell also includes a positive electrode, a negative electrode, and an alkaline electrolyte, all disposed in the container. A pressure relief mechanism is formed in the end wall of the container for releasing internal pressure from within the container when the internal pressure becomes excessive. A cover is positioned on the end wall of the container to be in electrical contact therewith and extends over the pressure relief mechanism. The cover has a peripheral flange with a peripheral edge and a protruding nubbin having a contact surface and an upstanding wall between the contact surface and the peripheral flange. The cover is engaged with the end wall of the container at a location near the upstanding wall.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.